The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. When a semiconductor device, such as a metal-oxide-semiconductor field-effect transistor (MOSFET), is scaled down through various technology nodes, high k dielectric material and metal are adopted to form a gate stack. In addition, to further enhance the performance of MOSFET devices, stress may be introduced in the channel region of a MOSFET device to improve carrier mobility. For example, the strained structures utilizing epitaxial structure may be used to enhance carrier mobility.